1. Field of the Invention
The invention relates to a display unit and an electronic device including the display unit.
2. Description of the Related Art
There have been suggested various display units including a display panel displaying images in accordance with received display-drive signals.
In such display units, a display panel is often covered at display screen thereof, that is, a surface thereof facing a user with an electrically conductive cover panel such as a touch panel.
As an example of such display panels, a conventional liquid crystal display device including a display panel covered with a touch panel is illustrated in FIG. 1.
As illustrated in FIG. 1, a conventional liquid crystal display device 100 having a touch panel is comprised of a liquid crystal display panel 110, and a touch panel 120 arranged above a display screen of the liquid crystal display panel 110.
The liquid crystal display panel 110 is comprised of a first substrate 111, a second substrate 112 arranged in parallel with the first substrate 111, a common electrode 113 formed on one of the first and second substrates 111 and 112 (for instance, the second substrate 112), and drain and gate electrodes (not illustrated) both formed on the other substrate. A common-electrode signal (COM) as a signal for driving the liquid crystal display panel 110 is input into the common electrode 113. The common-electrode signal has a high level of 5V and a low level of 0V, and a frequency of 10 kHz.
The touch panel 120 is comprised of a first substrate 121, a second substrate 122 arranged in parallel with the first substrate 121, a first electrically conductive transparent film 123 formed on a surface the first substrate 121 facing the second substrate 122, and a second electrically conductive transparent film 124 formed on a surface the second substrate 122 facing the first substrate 121.
The first and second electrically conductive transparent films 123 and 124 are adhered to each other at margins thereof through a double-sided adhesive 125 such that a certain gap between the first and second electrically conductive transparent films 123 and 124 is maintained.
Touch-panel drive signals XR, XL, YU and YD as signals for driving the touch panel 120 are input into the first and second electrically conductive transparent films 123 and 124. Each of the touch-panel drive signals XR, XL, YU and YD has a high level of 5V and a low level of 0V, and a frequency of 100 kHz.
The touch panel 120 and the liquid crystal display panel 110 are adhered to each other at margins thereof such that a certain gap is maintained therebetween.
The above-mentioned conventional liquid crystal display device 100 is accompanied with unintentional sound.
FIG. 2 is a schematic view of the conventional liquid crystal display device 100, used for explaining the mechanism of generation of unintentional sound, FIG. 3 is a graph showing a relation between voltages of the common electrode 113 and the first and second electrically conductive transparent films 123 and 124, and lapse of time, and FIG. 4 is a graph showing a relation between a direction of an electric field generated between the common electrode 113 and the first and second electrically conductive transparent films 123 and 124, and lapse of time. In FIG. 4, a direction towards the first and second electrically conductive transparent films 123 and 124 from the common electrode 113 is defined as a positive direction.
When a user makes touch with the touch panel 120 to carry out a certain action, the first substrate 121 of the touch panel 120 may make contact with the second substrate 112 of the liquid crystal display panel 110. As a result, when the first substrate 121 of the touch panel 120 leaves the second substrate 112 of the liquid crystal display panel 110, electric charges “e” are accumulated on a surface of the second substrate 112 of the liquid crystal display panel 110 due to a phenomenon called “peeling charge”, as illustrated in FIG. 2. In brief, the second substrate 112 of the liquid crystal display panel 110 is charged at a surface thereof because of a touch to the touch panel 120 made by a user.
The common electrode signal COM to be input into the liquid crystal display panel 110 and the touch-panel drive signals XR, XL, YU and YD to be input into the touch panel 120 commonly have a high level of 5V and a low level of 0V, but have frequencies different from each other. That is, the common electrode signal COM and the touch-panel drive signals XR, XL, YU and YD are asynchronous with each other.
Thus, as illustrated in FIG. 3, the first and second electrically conductive transparent films 123 and 124 of the touch panel 120 and the common electrode 113 of the liquid crystal display panel 110 have voltages different from each other. In addition, since a voltage of the first and second electrically conductive transparent films 123 and 124 is sometimes higher and sometimes lower than a voltage of the common electrode 113, as illustrated in FIG. 3, there is generated an electric field directing differently with the lapse of time, between the first and second electrically conductive transparent films 123 and 124, and the common electrode 113, as illustrated in FIG. 4.
Hence, if the liquid crystal display panel 110 is charged at a surface thereof due to the above-mentioned “peeling charge”, electrostatic force acts on electric charges “e” in accordance with a direction of the electric field.
Specifically, when the electric field is in a direction towards the first and second electrically conductive transparent films 123 and 124 from the common electrode 113, the electric charges “e” are attracted in the direction, and vice versa.
As a result, the liquid crystal display panel 110 is oscillated in accordance with a direction of the electric field.
Since a frequency at which the liquid crystal display panel 110 oscillates, that is, a cycle at which the electric field changes its direction is dependent on a frequency of the touch-panel drive signal or the common electrode signal, if those signals have a frequency within an audible band, a user would listen to the oscillation of the liquid crystal display panel 110 as “unintentional sound”.
FIG. 5 illustrates a conventional liquid crystal display device 200 comprised of a liquid crystal display panel 110, and a protection cover 210 covering a display screen of the liquid crystal display panel 110 therewith as a cover panel.
The above-mentioned “unintentional sound” occurs not only in the liquid crystal display device 100 including the touch panel 120, illustrated in FIG. 1, but also in the liquid crystal display device 200 including the protection cover 210, illustrated in FIG. 5.
This is because when the protection cover 210 having been compressed for some reason makes contact with the liquid crystal display panel 110, the above-mentioned “peeling charge” occurs at a surface of the liquid crystal display panel 110, and electrostatic force acts on the resultant electric charges “e” in accordance with a direction of electric field generated due to the common electrode signals.
For instance, Japanese Patent Application Publication No. 2002-341372 has suggested a solution to the problem caused by the liquid crystal display panel 110 being charged.
Specifically, the Publication suggests a liquid crystal display device comprised of a liquid crystal display panel, and an electrically conductive transparent film formed on a display screen of the liquid crystal display panel, the electrically conductive transparent film being electrically grounded.
However, the suggested liquid crystal display device cannot actually solve the problem of “unintentional sound”. To the contrary, “unintentional sound” would be higher in the suggested liquid crystal display device than in other liquid crystal display devices.